1. Field of the Invention
The invention pertains generally to circuit interrupters and, more particularly, to corded leakage-current detection and interruption apparatus.
2. Background Information
Ground fault circuit interrupters (GFCIs) include, for example, ground fault circuit breakers, ground fault receptacles and cord mounted ground fault protection devices. GFCIs and arc fault current interrupters (AFCIs) are well known in the art. Examples of ground fault and arc fault circuit breakers are disclosed in U.S. Pat. Nos. 4,081,852; 5,260,676; 5,293,522; 5,896,262; and 5,892,593.
In ground fault circuit breakers, for example, an electronic circuit typically detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a trip solenoid, which unlatches an operating mechanism, often through deflection of the armature of a thermal-magnetic trip device. Ground fault circuit breakers include both Class A (e.g., ground fault current of about 5 mA for people protection) and equipment protective devices (e.g., ground fault current of about 30 mA; of about 20 to about 100 mA).
A typical GFCI includes an operational amplifier which amplifies the sensed ground fault signal and applies the amplified signal to a window comparator which compares it to positive and negative reference values. If either reference value is exceeded, then a trip signal is generated. A common type of ground fault detection circuit is the dormant oscillator detector. This detector includes a first sensor coil through which the line and neutral conductors of the protected circuit pass. The output of the first sensor coil is applied through a coupling capacitor to the above-described operational amplifier followed by the window comparator. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal.
The dormant oscillator detector typically includes a second sensor coil. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils which causes the amplifier to oscillate, thereby resulting in the generation of the trip signal.
A leakage-current detection and interruption (LCDI) device is provided in a power supply cord or cord set that senses leakage current flowing between or from the integral cord conductors and interrupts the circuit at a predetermined level of leakage current. For example, cord protection has recently been required for air conditioners per UL 1699.
U.S. Pat. No. 5,148,344 discloses an appliance leakage current interrupter (ALCI) comprising a ground fault detecting circuit and a trip mechanism that is automatically actuated in response to the presence of a ground fault.
U.S. Pat. No. 5,642,248 discloses a phase wire, a neutral wire and a ground wire, which are utilized within an electrical extension or power cord with built-in safety protection. The power cord also includes a conductive braid (i.e., fine mesh shield) which is electrically connected to the ground wire at a receptacle end. The phase wire is electrically connected to the anode of a diode within a plug housing including a GFCI. The cathode of the diode is electrically connected through a resistor to the anode of a light emitting diode (LED), also contained within the housing. The cathode of the LED is electrically connected to the conductive braid shield. The LED is illuminated when the braid shield is intact with a current flowing from the load end side of the GFCI through the diode and a series resistor to the ground wire at the receptacle. If the braid shield were to become discontinuous, then the LED would be extinguished depicting a lack of protection by that conductive braid shield. Because the braid shield is electrically connected to the ground wire, excess ground fault or leakage current is passed to ground while the GFCI detects an imbalance within the phase or neutral wires and trips to open circuit the electrical path through the cord.
U.S. Patent Application Publication No. 2003/0066670 discloses an electrical cable with a grounding device. A bipolar cable includes two conductors (active, live or neutral conductors), an insulating coating on each of the conductors, and an external protection conductor (woven with wire, strands, plait, strap or mesh, which surrounds it concentrically in the form of a sheath) around each of the individual cores. An outer insulating sheath covers the cable.
U.S. Patent Application Publication Nos. 2004/0070895 and 2004/0070899 disclose an LCDI extension cord with cord diagnostics and/or inadvertent ground-to-neutral detection. Three electrical conductors comprise a three-wire conductor having an alternating current (AC) power source compatible plug at the source end, a control circuit and interrupter circuit contained in the plug, and a detector contained within the appliance. A fourth electrical conductor can be a single un-insulated wire which runs substantially parallel with the other wires in the cord, or it can be a spiral wound wire or a conductive shield which surrounds the insulated phase, neutral and ground conductors in the cord. This shield may be a conductive shield which surrounds the various conductors of the extension cord, or may be one or more wires in substantially parallel relationship with the other wires in the cord, or may be one or more wires which surround the various wires in the extension cord. In one embodiment, the LCDI circuit is located in the plug of an extension cord and has a shield integrity indicator in the receptacle of the extension cord. In another similar embodiment, the shield integrity indicator is in the plug and there is a return wire from the shield to that shield integrity indicator in the cord. In another similar embodiment, a shield integrity indicator test switch is in the extension cord plug and there is a return wire from the shield to that test switch in the cord.
There is room for improvement in corded leakage-current detection and interruption apparatus, and in detection mechanisms for cord faults in such apparatus.